The usual practice for continuous casting of molten steel comprises pouring molten steel received in a tundish from a ladle through a monolithic molten steel pouring nozzle fitted to a bottom wall of the tundish into a vertical mold arranged below the molten steel pouring nozzle to form a cast strand, and continuously withdrawing the thus formed cast strand in the form of a long strand.
Since a lower end portion of the above-mentioned monolithic molten steel pouring nozzle is immersed into molten steel in the vertical mold, the lower end portion suffers from particularly serious erosion. As a result, the monolithic molten steel pouring nozzle poses an economic problem of a relatively short service life.
For the purpose of solving the above-mentioned economic problem, therefore, a molten steel pouring nozzle comprising an upper nozzle and a lower nozzle has recently been employed. According to the molten steel pouring nozzle of this type, even if a lower end portion of the lower nozzle is immersed into molten steel in a vertical mold and seriously eroded, it suffices to replace only the lower nozzle, so that the above-mentioned problem is solved.
In the above-mentioned molten steel pouring nozzle, for the purpose of preventing an open air from penetrating through an interface between the upper nozzle and the lower nozzle into a vertical path for molten steel formed by the bores of both of the upper and lower nozzles and thus preventing oxidation of molten steel flowing through the vertical path, it is the usual practice to improve sealing property of the above-mentioned interface by arranging sealing members comprising a ceramic fiber or an alumina-based castable refractory on the interface between the upper nozzle and the lower nozzle.
By such a means, however, it is impossible to obtain a sufficient sealing property of the interface between the upper nozzle and the lower nozzle.
In order to solve this problem, a molten steel pouring nozzle capable of giving a sufficient sealing property of the interface between the upper nozzle and the lower nozzle is disclosed in Japanese Patent Provisional Publication No. 62-279,072 dated Dec. 3, 1987 (hereinafter referred to as the "prior art"). The molten steel pouring nozzle A of the prior art is described below with reference to FIG. 1.
FIG. 1 is a schematic vertical partial sectional view illustrating the molten steel pouring nozzle A of the prior art, which is to be fitted to a bottom wall of a molten steel receiving vessel such as a ladle or a tundish. As shown in FIG. 1, the molten steel pouring nozzle A of the prior art comprises an upper nozzle 1 made of a refractory, a lower nozzle 4 made of a refractory, and an open air shutoff means 11.
The upper nozzle 1 has along the center axis thereof an upper bore 2, and an upper end portion of the upper nozzle 1 is to be vertically inserted into an opening provided in a bottom wall of a tundish (not shown) as a molten steel receiving vessel. A lower end portion 3 of the upper nozzle 1 is tapered.
The lower nozzle 4 has along the center axis thereof a lower bore 5 aligning with the upper bore 2, and has, on an upper end portion 6 thereof, a recess 7 for receiving the lower end portion 3 of the upper nozzle 1. The recess 7 is formed coaxially with the lower bore 5 and communicates therewith. An annular notch 10 for receiving a sealing member 15 described later as one of the components of the open air shutoff means 11 is formed throughout the entire circumference of the upper end portion 6 of the lower nozzle 4.
The open air shutoff means 11 comprises a steel shell 12 covering the upper end portion 6 of the lower nozzle 4, a sealing member 15 made of a porous refractory arranged between the upper end portion 6 of the lower nozzle 4 and the steel shell 12, and an annular equalizing chamber 18 defined by the steel shell 12 and the sealing member 15.
The steel shell 12 comprises a side surface 12b formed into a cylindrical shape so as to cover the circumference of the upper end portion 6 of the lower nozzle 4, and an upper surface 12a welded to the upper end of the side surface 12b. The upper surface 12a of the steel shell 12 has, at the center thereof, an opening 13 for receiving the lower end portion 3 of the upper nozzle 1, and the opening 13 has a size slightly larger than that of the recess 7 of the lower nozzle 4. The side surface 12b of the steel shell 12 has, at the upper end portion thereof, an inert gas feed port 14. The side surface 12b of the steel shell 12 is stuck to the upper end portion 6 of the lower nozzle 4 by means of an adhesive 19 such as mortar, and the upper surface 12a of the steel shell 12 is welded to the upper end of the side surface 12b of the steel shell 12 after the sealing member 15 made of a porous refractory has been fitted to the upper end portion 6 of the lower nozzle 4.
The sealing member 15 made of a porous refractory is formed into an annular shape between the upper end surface of the lower nozzle 4 and the upper surface 12a of the steel shell 12 so as to cover the lower end portion 3 of the upper nozzle 1. More specifically, the sealing member 15 is arranged between the upper end surface of the lower nozzle 4 and the upper surface 12a of the steel shell 12, and has a bore 15' for receiving the lower end portion 3 of the upper nozzle 1 at the center of the sealing member 15. The bore 15' of the sealing member 15 therefore receives the lower end portion 3 of the upper nozzle 1 in cooperation with the recess 7 of the lower nozzle 4. The sealing member 15 has, throughout the entire circumference thereof, an annular notch 16 for forming the annular equalizing chamber 18 in cooperation with the side surface 12b of the steel shell 12. Furthermore, the sealing member 15 has, on the lower end portion thereof, an annular projection 17 engaging with the annular notch 10 of the lower nozzle 4. The sealing member 15 is secured to the upper end surface of the lower nozzle 4 by means of an adhesive such as mortar in the state that the annular projection 17 of the sealing member 15 engages with the annular notch 10 of the lower nozzle 4. In this state, the annular equalizing chamber 18 communicates with the inert gas feed port 14 of the steel shell 12, and the inert gas feed port 14 is connected through a pipe (not shown) to an inert gas source (not shown).
According to the molten steel pouring nozzle A of the prior art having the construction as described above, an inert gas supplied from the inert gas source (not shown) through the inert gas feed port 14 into the annular equalizing chamber 18 flows through numerous pores in the sealing member 15 made of a porous refractory, is ejected toward the lower end portion 3 of the upper nozzle 1, and discharged to the open air through the gap between the edge of the opening 13 in the upper surface 12a of the steel shell 12 and the lower end portion 3 of the upper nozzle 1. As a result, the circumference of the lower end portion 3 of the upper nozzle 1 is filled with the inert gas, thus preventing the open air from penetrating through an interface 9 between the lower end portion 3 of the upper nozzle 1 and the recess 7 of the lower nozzle 4 into a vertical path 8 for molten steel formed by the upper bore 2 of the upper nozzle 1 and the lower bore 5 of the lower nozzle 4.
However, the above-mentioned molten steel pouring nozzle A of the prior art has the following problems. The sealing member 15, being made of the porous refractory, is inferior in spalling resistance to the upper nozzle 1 and the lower nozzle 4. Therefore, cracks may occur in the sealing member 15 due to sudden temperature changes during operation. If cracks occur in the sealing member 15, it becomes impossible to uniformly eject inert gas toward the lower end portion 3 of the upper nozzle 1, thus allowing an open air to penetrate through the interface 9 between the lower end portion 3 of the upper nozzle 1 and the recess 7 of the lower nozzle 4 into the vertical path 8 for molten steel, and hence leading to a lower quality of the cast steel strand. In addition, because of the large number of parts of the molten steel pouring nozzle A of the prior art, fabrication of the individual parts and the assembly operations thereof are complicated and tend to result in a high manufacturing cost.
The above-mentioned problems are posed also when pouring molten steel received in a ladle into a tundish through the molten steel pouring nozzle A of the prior art fitted to a bottom wall of the ladle.
Under such circumstances, there is a strong demand for development of a molten steel pouring nozzle which ensures a sufficient sealing property of an interface between the upper nozzle and the lower nozzle, thus preventing deterioration of the quality of a cast steel strand caused by the penetration of an open air through the above-mentioned interface, and is manufacturable at a low cost, but such a molten steel pouring nozzle has not as yet been proposed.